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LTE_MIMO

By Carolyn Jenkins,2014-05-26 12:29
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LTE_MIMO

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     LTE ?ÕÖÐ?Ó?Ú??ÊõÓëÐÔÄÜ

     MIMO

     Ìïèº

     Confidential Information

     Multichannel Communications

     Multiplexing and Multiple Access > Multiplexing:

     divide one stream into several branches and transmit over several (independent) channels in

     ? ? ? Time: time-division multiplexing (TDM) Frequency: frequency-division multiplexing (FDM) Code: code-division multiplexing (CDM) Space: space-division multiplexing (SDM)

     > Different multiplexing can be employed jointly, e.g.,

     MIMO-OFDM

     > Multiple access = multiple-user multiplexing

     Scheduling and signaling are different ? All multiplexing schemes support multiple access

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     OFDMA

     Time-Frequency Radio Resource

     Frequency

     User #1 payload User #2 payload User #3 payload User #4 payload User

    #5 payload Pilots

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     OFDM symbol duration

     Time

     Confidential Information

     MIMO: Introduction

     Spatial Dimension

     > Why spatial dimension?

     Spatial dimension offers unique features ? Well established techniques

     Array processing ? Diversity combining

     Theoretical breakthrough in MIMO (early 1990s)

     > Why MIMO

     Increasing channel capacity ? Increasing robustness

     > MIMO Classification

     Spatial Multiplexing ? Spatial Diversity ? Beam-forming

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     MIMO: Introduction

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     MIMOÊ?ÓÃ

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     SIMO and MISO

     Beamforming and Diversity Combining

     > Similarities

     Both do combining ? Both apply to SIMO and MISO

     > Differences

     Array spacing/correlatin

     BF applies to correlated array ? Diversity applies to uncorrelated array

     Gains

     BF achieves array gain (aka, BF gain) ? Increasing signal strength ? Diversity achieves diversity gain ? Reducing signal fluctuation

     > Hybrid configuration

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     MISO

     Transmit Diversity (TxD)

     > Delay Diversity

     Increasing frequency selectivity

     Large delay => frequency diversity ? Small delay => multiuser diversity

     Cyclic Delay Diversity (CDD)

     Increasing frequency selectivity without increasing delay spread ? Applicable to OFDM ? Applicable to SIMO, i.e., Rx-CDD

     > Alamouti Scheme

     Transmit transformed signals over different antennas and slots ? Slots could be time or frequency

     Space-Time Transmit Diversity (STTD) ? Space-Frequency Block Code (SFBC)

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     MIMO: Single-User

     Spatial Multiplexing (SM)

     > What is SM?

     Multi-stream transmission to single user, a.k.a. Single User (SU) MIMO

     TX Data stream s1 Data stream s2 Tx Proc.

     h11

     RX Data stream s1 Rx Proc. Data stream s2

     > Why SM?

     Increasing user throughput by transmitting different data from different antennas

     h 12 h2

     1

     h22

     MIMO channel Data stream s1 Data stream s2 Equivalent MIMO Channel

     > How?

     Creating orthogonal channels (streams, layers) via Rx processing and/or Tx processing

     Data stream s1 Data stream s2

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     MIMO: Single User

     Transceiver Structure for SM

     > Non-precoded SM

     Bell Labs Layered Space-Time (BLAST) architecture ? Rx: linear (ZF, MMSE), nonlinear (SIC), ML

     > Precoded SM

     Optimal Tx-Rx is multiple eigen-beamforming

     High signalling overhead

     Codebook-based precoding

     Quantized version of optimal precoding, aka. Grassmannian precoding ? Reduced overhead ? Codebook design ? Structured matrices (DFT, Householder) ? Computer exhaustive search

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     MIMO: Multiple-User

     MU-MIMO

     > What is MU-MIMO?

     multi-stream transmission to multiple users

     > Why MU-MIMO?

     increasing sector throughput

     > How?

     Spatial Domain Multiple Access (SDMA) forming multiple beams to multiple users

     A special case of MU-MIMO using correlated array ? Multiuser scheduling/pairing

     Codebook-based implementation

     Requires extra processing, e.g. ZF

     Opportunistic beamforming

     open-loop

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     MIMO: Multiple-User

     MU-MIMO Downlink

     [v 1 ?d ? v 2 ]? 1 ? ?d 2 ?

     [h1 ?d ? h2 ] ? 1 ? ?d 2 ?

     1

     X1

     +

     ) x4 3+ +x (x2

     +x4) X2 + (x3

     X3 +

     X4

     (x4)

     Beam index, CQI

     Channel state info

     Channel state info

     ? ? ?

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     SDMA Spatial matchedfilter Low overhead Limited precoder Performance ?ý

     ZF-Precoding ? Simple linear precoding ? Power loss

     Dirty Paper Coding ? Utilizing interference ? Optimal performance

     Confidential Information

     MIMO: Precoding

     Summary

     > Precoding was first proposed for single-antenna systems in early 1970??s by Tomlinson and Harashima, known as TH precoding

     TH precoding is essentially pre-equalization (DFE) at Tx

     > Linear precoding for MIMO

     Unifies Tx linear processing for most existing MIMO schemes

     ? ? ? ? TX-BF and eigen-BF Delay diversity SU-MIMO MU-MIMO Antenna selection/switching/grouping

     It is essentially (multiple) beamforming, allowing processing in beam space

     > Dirty Paper Coding

     Nonlinear precoding ? One simple implementation can be viewed as SIC at Tx

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     MIMO-OFDM(A)

     A brief introduction

     > Integration of OFDM with MIMO

     OFDM transmission over MIMO layers (streams)

     > Space, time, frequency and multiuser diversity > Multiplexing in space and frequency domains > Multiple access in both space and frequency

domains > Flexible resource management

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     Multi-antennas transmission Techniques

     > MIMO multiplexing (to data channel)

     Baseline:2-by-2 MIMO (Downlink) 1-by-2 SIMO (Uplink) Maximum of 4 antennas ? Supports single user-MIMO and multi user-MIMO

     > MIMO Diversity (transmit diversity) (to data and control channel)

     Apply appropriate transmit diversity scheme to each downlink /uplink channel

     > Adaptive beam-forming (to data channel)

     Beneficial in increasing cell edge user data rate and coverage

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     DL MIMO Processing Chain

     Layer Mapping

     Precoding

     (q b(q) (0),????,b(q) (Mbit) ?1)

     (q ) d = d ( q) (0) ???? d ( q) (M symb?1 )

     [

     ]

     >Code word: A sequence of bits or symbols transmitted on the physical channel in one subframe. >Up to two code words can be transmitted in one subframe:

     One code word: ? Single Tx antenna configuration ? Multiple antenna configuration using transmit diversity precoding ? Two code words: ? Multiple antenna configuration using spatial multiplexing precoding

     >Modulation symbols of each code words are mapped to one or several layers

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     LTE??ÐŵÀÌìÏß??Ê?

     > LTE FDDÐ?ÒéÄ?Ç?Ö??ÖµÄ×î?óÌìÏßÊýΪ?ùÕ?4????ÖÕ?Ë2???? > TDDÐ?Òé?ÉÖ??Ö?óÓÚ4ÌìÏßµÄÌìÏßÅäÖÃ??

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     Layer Mapping

     > For SM, mapping 1 or 2 code words to 1, 2 , 3 or 4 layers( virtual antenna ) > For TxD, mapping 1 code word to 2 or 4 layers ( virtual antenna )

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     CW

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     Layer Mapping ?C Spatial Multiplexing

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     Layer Mapping ?C Spatial Multiplexing

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     Layer Mapping ?C Transmit Diversity

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     Downlink MIMO, Tx. Diversity and Beamforming

     > Default assumption is 2 Tx and 2 Rx antennas for DL > For DL user traffic, both closed-loop MIMO and open-loop MIMO are supported. > Closed-loop MIMO uses codebook based precoding with zero and small- delay CDD. Dynamic rank adaptation is supported. > Open-loop MIMO uses precoding with large-delay CDD for rank >1 transmission and uses transmit diversity for rank = 1 transmission. Dynamic rank adaptation is supported. > SFBC (Space-Frequency Block Coding) based scheme is used as transmit diversity for traffic and control channels. > SFBC is used for 2-tx and SFBC+FSTD (frequency-shift time diversity) is used for 4-tx

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     Downlink MIMO, Tx. Diversity and Beamforming

     > Closed-loop MIMO

     eNB x CQI, PMI, RI Data UE

     > Closed-loop MIMO uses codebook based precoding with zero and small CDD.

     For 2-tx system, the codebook consists of 6 rank-1 codewords and 3 rank-2 codewords ? For 4-tx system, the codebook consists of 16 codewords for each rank (rank-1 to rank-4)

     > For closed-loop MIMO, UE should feedback channel quality indication (CQI), Precoding matrix index (PMI) and rank indicator (RI)

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     Downlink MIMO, Tx. Diversity and Beamforming

     > Closed-loop MIMO: Precoding detail with zero and small CDD

     y ( 0 ) (i ) ? ? x ( 0 ) (i ) ? ? ? ? ? ?? ? = D ( k i )W ( i ) ? ?? ? ? ? x (?Ô ?1) ( i ) ? ? y ( P ?1) ( i ) ? ? ? ? ?

     Set of antenna ports used Number of layers

    

     D(ki )

     No CDD 0 Small delay

    

     1 2

     {0,1} {0,1,2,3}

     0 ? ?1 ?0 e? j2?Ð?ki ??Ä ? ? ?

     0 0 0 ? ?1 ?0 e? j2?Ð?ki ??Ä 0 0 ? ? ? ? j 2?Ð ?ki ?2?Ä ?0 0 e 0 ? ? ? 0 0 0 e? j2?Ð?ki ?3?Ä ? ?

     Confidential Information

     2?Ç

     1

     0

     1?Ç

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     Downlink MIMO, Tx. Diversity and Beamforming

     > Open-loop MIMO Data x CQI, RI > Open-loop MIMO uses precoding with large-delays CDD for rank > 1 transmission and transmit diversity for rank = 1 transmission.

     For 2-tx system, a fixed rank-2 codeword is used for rank-2 transmission ? For 4-tx system, the precoder cyclically assign codeword from a subset of 4 codewords (each for rank=2,3,4) to a number of neighboring data resource elements, for rank > 1 transmission ? For rank =1 transmission, SFBC is used for 2-tx and SFBC+FSTD is used for 4-tx system.

     eNB

     UE

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     Downlink MIMO, Tx. Diversity and Beamforming

     > Open-loop MIMO: Precoding detail with large delay CDD for rank>1 transmission

     y (0 ) (i ) ? ? ? ?? ? ? = W ( i ) D ( i )U ? y ( P ?1) ( i ) ? ? ?

     Number of layers ?Ô 1 2

     x (0 ) (i ) ? ? ? ?? ? ? ? x (?Ô ? 1 ) ( i ) ? ? ?

     D(i)

     U

     [1]

     1 ? 1 ?1 ? ? j 2?Ð 2 ? 2 ?1 e ? 1 1 ? ?1 1 ? ? j 2?Ð 3 e ? j 4?Ð 3 ? ? ?1 e 3? ? j 4?Ð 3 ? j8?Ð 3 ? e ? ?1 e 1 1 1 ? ?1 ? ? j 2?Ð 4 e ? j 4?Ð 4 e ? j 6?Ð 4 ? 1 ?1 e ? ? j 4?Ð 4 ? j 8?Ð 4 ? j12?Ð 4 ? e e 2 ?1 e ? ? 1 e ? j 6?Ð 4 e ? j12?Ð 4 e ? j18?Ð 4 ? ?

     [1]

     0 ? ?1 ?0 e ? j 2?Ði 2 ? ? ?

     3

     0 0 ? ?1 ?0 e ? j 2?Ði 3 0 ? ? ? ? j 4?Ði 3 ? ?0 0 e ? ?

     0 0 0 ? ?1 ?0 e ? j 2?Ði 4 0 0 ? ? ? ? j 4?Ði 4 ?0 0 e 0 ? ? ? 0 0 0 e ? j 6?Ði 4 ? ?

     4

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     Downlink MIMO, Tx. Diversity and Beamforming Downlink ?C Transmit diversity

     SFBC for 2-tx system Freq

     Antenna 0

     a1 a2

     Antenna 1

     * * ? a2 a1

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     Downlink MIMO, Tx. Diversity and Beamforming Downlink ?C Transmit diversity

     SFBC+FSTD for 4-tx system Freq

     Antenna 0

     a1 a2

     Antenna 1

     a3 a4

     Antenna 2

     * * ? a2 a1

     Antenna 3

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     * * ? a4 a3

     Confidential Information

     Downlink MIMO, Tx. Diversity and Beamforming

     > Beamforming (BF) operation

     Configured by higher layer in PDSCH ? UE-specific RS (antenna port 5) are transmitted and used as phase reference for data decoding. ? Single antenna transmission is assumed for PDSCH in BF operation. ? Cell-specific RS is maintained and UE could only use cell-specific RS from antenna ports 0 and 1 for PDCCH decoding.

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     Downlink MIMO, Tx. Diversity and Beamforming >UE feedback to support downlink MIMO

     RI, CQI and PMI can be feedback on PUCCH (periodic) and PUSCH (aperiodic and periodic) ? Rank indicates how to decode CQI/PMI before next new rank is available ? CQI feedback

     A 4-bit CQI table based on MCS is used in UE

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     Uplink Multi-Antenna Techniques

     > Default assumption is 1 Tx and 2 Rx antennas for UL > Maximum Ratio

Combining (MRC) > Spatial multiplexing

     MU-MIMO (assuming each UE transmit from one antenna) on UL is supported. Mainly used as Virtual MIMO (V-MIMO) ? SU-MIMO (each UE transmits with two or more antennas) is not supported in Rel. 8

     > Spatial-Division Multiple Access (SDMA)

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     Uplink Multi-Antenna Techniques

     > Uplink MU-MIMO

     To support uplink MU-MIMO, eNB schedules two or more UE transmit on the same uplink resources, each with one transmit antenna. ? The uplink demodulation reference signals (DMRS) used by each UE can be separated by using different cyclic shifts to guarantee good channel estimation performance. ? This cyclic shift indication (CSI) is signaled on uplink grant. ? The different cyclic shifts used by each UE is used to map ACK/NACK from each UE to different PHICH channel.

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     Overhead in LTE

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     ?ùÓÚ×ÔÊÊÓ???Êø??ÐεÄ?ÇÂë????À???Ê?

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     ?ùÓÚ??Êø??ÐεÄÂë????À???Ê?

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     MIMO-BF

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     MIMO-BF

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     Outline

     Guardbands CP Overhead Downlink Overhead

     PCFICH, PHICH, PDCCH, PBCH, PSC, SSC, RS

     Uplink Overhead

     Control, PUCCH, RS, SRS

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     Guard Bands

     > 10% for most of the channel bandwidths > Reaches 23% in the lowest 1.4 MHz bandwidth > 21% for 1.6 MHz > See ??LTE Bandwidths?? slide for details

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     LTE Bandwidths

     Channel Bandwidth [MHz] Duplexing Scheme Transmission Bandwidth configuration Guardband # RB??s # SC??s MHz MHz % 1.4 FDD 6 72 1.08 0.32 23% 1.6 TDD 7 84 1.26 0.34 21% 3 FDD 15 1802.7 0.3 10% 3.2 TDD 16 192 2.88 0.32 10% 5 FDD/ TDD 25 300 4.5 0.5 10% 10 FDD/ TDD 50 600 9.0 1.0 10% 15 FDD/ TDD 75 900 13.5 1.5 10% 20 FDD/ TDD 100 1200 18.0 2.0 10% 1.6 & 3.2 MHz B/Ws could be dropped

     Example: ? Channel B/W = 5 MHz (includes guardband) ? This B/W profile has 25 RB??s, so ??Transmission B/W Configuration?? = 25*180 = 4.5 MHz (this is the usable B/W) ? 0.5 MHz is left as a guardband ? Assume a user needs only 10 RB??s, then the ??Transmission B/W?? = 10*180 = 1.8 MHz

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     CP Overhead

     > 7 OFDM symbols/slot for short CP > (6 OFDM symbols/slot for long CP) > Useful OFDM symbol duration = Tb = 1/15kHz = 66.67 usec > CP duration = # samples/FFT size * Tb

     5.21 usec in symbol 0 ? 4.69 usec in symbols 1-6,

     ?2 ?4 C P ? =? ?8 ?1 ? ? ? ?1 ?3 C P ? =? ?7 ?1 ? ?

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     1 0 0 0 0 6 0

     f o r F F T 1 2 8 f o r F F T 2 5 6 f o r F F T 5 1 2 f o r F F T 1 0 2 4 f o r F F T 2 0 4 8 , s y m b ol : 0

     > Total symbol duration:

     71.88 usec (in symbol 0) ? 71.35 usec (in symbols 1-6)

     f o r F F T 1 2 8 8 6 2 4 4 f o r F F T 2 5 6 f o r F F T 5 1 2 f o r F F T 1 0 2 4 f o r F F T 2 0 4 8 , s y m b ols : 1,2,3,4,5, 6

     > Slot duration = 71.88+6*71.35 = 500 usec > CP overhead = (5.21+6*4.69)/500 = 6.67% > User Tput is reduced by 6.67%

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     Downlink Overhead

     > PCFICH (Physical Control Format Indicator Channel)

     ? ? ? Carries information about the number of OFDM symbols (1, 2 or 3) used for transmission of PDCCHs in a subframe. Transmitted in the 1st symbol of each subframe (1st slot) Uses 16 subcarriers, scattered in groups of 4 REs QPSK modulated

     > PHICH (Physical Hybrid ARQ Indicator Channel)

     Carries ACK/NAK bits - when a transport block is received on the PUSCH, a CRC is checked in the eNB. Then, a downlink ACK/NACK as a result of CRC OK/NOK is explicitly transmitted on PHICH at a certain time. ? Transmitted in the first OFDM symbol of the subframe (1st slot). ? DL

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